Abstract:

The present invention relates to a method for the calibration of a
position determination system of a rear axle steering actuator for a
motor vehicle. The rear axle steering actuator has an actuator element
which can be driven by a rotary movement of a rotor to a translation
movement and whose geometrical center position is determined by means of
a reference measurement. The position determination system includes a
linear sensor and a rotary sensor. During the calibration of the position
determination system, a piece of calibration information is generated
which includes a piece of zero point information of the linear sensor and
a piece of sector information. The measurement range of the rotary sensor
is divided into at least two sectors. The sector information identifies
that angle at which the angular position of the rotor defined by the
rotary sensor lies when the actuator element is arranged in its
geometrical center position. The calibration information is stored in the
linear sensor.

Claims:

1. A method for the calibration of a position determination system of a
rear axle steering actuator for a motor vehicle which has an actuator
element (28) which can be driven by a rotary movement of a rotor (16) to
a translation movement and whose geometrical center position is
determined by means of a reference measurement,wherein the position
determination system includes a linear sensor (50) and a rotary sensor
(52);wherein a piece of calibration information is generated during the
calibration which includes a piece of zero point information of the
linear sensor (50) and a piece of sector information (52);wherein the
measurement range of the rotary sensor (52) is divided into at least two
sectors (Sec 0-Sec 3) and the sector information identifies that sector
(Sec 0-Sec 3) in which the angular position (Mi) of the rotor (16)
detected by the rotary sensor (52) lies when the actuator element (28) is
arranged in its geometrical center position;and wherein the calibration
information is stored in the linear sensor (50).

2. A method in accordance with claim 1, characterized in that the
respective angular region covered by the sectors (Sec 0-Sec 3) is larger
than the angular resolution of the rotary sensor (52).

3. A method in accordance with claim 1, characterized in that the linear
sensor (50) includes a non-volatile memory section in which at least a
part of the calibration information is stored.

4. A method in accordance with claim 1, characterized in that a respective
predetermined frequency of an output signal of the linear sensor (50) is
associated with the sectors (Sec 0-Sec 3); and in that the sector
information is stored in the linear sensor (50) by a selection of the
corresponding frequency.

5. A method in accordance with claim 1, characterized in that the
reference measurement respectively includes at least one measurement in
at least two defined reference positions, with each measurement including
at least one measured value determination of at least one sensor (50,
52).

6. A method in accordance with claim 1, characterized in that the
reference measurement includes measurements by means of a third position
sensor which is in particular based on an optical measuring process.

7. A method in accordance with claim 1, characterized in that the actuator
element (28) is moved into the geometrical center position;and in that
the zero point information of the linear sensor (50) and the sector
information are determined there.

8. A method in accordance with claim 1, characterized in that the
geometrical center position is determined by the formation of a
geometrical average of measurements in two maximum excursions (P1, P3) of
the actuator element (28).

9. A method in accordance with claim 8, characterized in that the maximum
excursions (P1, P3) of the actuator element (28) are defined by a
respective mechanical abutment.

10. A method in accordance with claim 8, characterized in that the
position of the actuator element (28) in the respective maximum excursion
(P1, P3) is checked by at least one repeat position determination and/or
is determined by averaging of at least two position determinations.

11. A method in accordance with claim 8, characterized in that the
reaching of the respective maximum excursion (P1, P3) is defined by
falling below a predetermined threshold value of the speed of the
actuator element (28).

12. A method in accordance with claim 8, characterized in that the sector
information is determined with reference to measured values of the rotary
sensor (52) determined in the maximum excursions (P1, P3) and to the
number of revolutions of the rotor (16) determined by means of the rotary
sensor (52) during the movement of the actuator element (28) between the
maximum excursions (P1, P3).

13. A method in accordance with claim 6, characterized in that a plurality
of measurements of the linear sensor (50) are carried out for the
position determination in different measurement positions of the actuator
element (28), with the location of the measurement positions relative to
the geometrical center position being known by measurements of the third
position sensor, and with at least one correction parameter and/or at
least one correction function being determined on the basis of the
deviations of the position determinations of the linear sensor (50) from
the measurement positions of the actuator element (28), which are taken
into the zero point information of the linear sensor (50).

14. A position determination system of a rear axle steering actuator for a
motor vehicle, wherein the rear axle steering actuator has an actuator
element which can be driven by a rotary movement of a rotor (16) to a
translation movement;wherein the position determination systemcan be
connected to a calibration unit for calibration; andincludes a rotary
sensor (52) and a linear sensor (50) which are connected to the rear axle
steering actuator; andwhereina geometrical center position of the
actuator element (28) between two maximum excursions (P1, P3) can be
determined by the calibration unit,the measurement range of the rotary
sensor (52) can be divided into at least two sectors (Sec 0-Sec 3),a
piece of calibration information can be generated which includes a piece
of zero point information of the linear sensor (50) and a piece of sector
information, wherein the sector information identifies that sector (Sec
0-Sec 0) in which the angular position (Mi) of the rotor (16)
determined by the rotary sensor (52) lies, when the actuator element (28)
is arranged in the geometrical center position; andthe calibration
information can be stored in the linear sensor (50).

15. A position determination system in accordance with claim 14,
characterized in that the respective angular range covered by the sectors
(Sec 0-Sec 3) can be divided so that it is larger than the angular
resolution of the rotary sensor (52).

16. A position determination system in accordance with claim 14,
characterized in that the linear sensor (50) includes a non-volatile
memory section in which at least one part of the calibration information
can be stored.

17. A position determination system in accordance with claim 14,
characterized in that the linear sensor (50) can be configured such that
the sector information is encoded by a frequency of an output signal of
the linear sensor (50) associated with the sector (Sec 0-Sec 3).

18. A position determination system in accordance with claim 14,
characterized in that the calibration unit includes a position sensor
which is in particular based on an optical measurement process, with the
position of the actuator element (28) being able to be determined
relative to a reference point by means of the position sensor.

19. A position determination system in accordance with claim 18,
characterized in that the calibration unit is made such that at least one
correction factor and/or at least one correction function can be
determined by a comparison of measurements of the linear sensor (50) and
of the position sensor in different positions of the actuator element
(28) which are taken into the zero point information of the linear sensor
(50).

20. A method for the operation of a rear axle steering actuator for a
motor vehicle having a position determination system in accordance with
claim 14,wherein the actuator element (28) is moved into a position
defined by the zero point information of the linear sensor (50) for
positioning in a neutral position in a first step; andwherein
subsequently the actuator element (28) is moved in a second step until
the angular position of the rotor (16) detected by the rotary sensor (52)
lies in the sector (Sec 0-Sec 3) identified by the sector information.

21. A method in accordance with claim 20 characterized in that the
actuator element (28) is moved in the second step until the angular
position of the rotor (16) corresponds to the bisectrix (M) of the
identified sector.

Description:

[0001]The present invention relates to a method for the calibration of a
position determination system of a rear axle steering actuator for a
motor vehicle.

[0002]In many actuators which are used in connection with a driving
dynamics regulation of a motor vehicle, actuation movements have to be
carried out precisely to achieve a respective desired effect
advantageously influencing the handling of the vehicle. A very high
absolute precision and resolution of the position sensor system is
required with respect to the position regulation and position monitoring
of an active rear axle steering, for example, to generate the suitable
reaction of the rear axle steering in dependence of a steering wheel
position.

[0003]Sensors which are able to determine the position of an element of
the actuator sufficiently accurately over its total excursion range are
frequently expensive. Optical sensors admittedly deliver the required
accuracy; however, they become dirty easily and are therefore not
sufficiently reliable.

[0004]A further aspect which has to be taken into account in the
conception of a rear axle steering actuator relates to the calibration of
the position determination system. Since every actuator and its sensors
have individual tolerances, every actuator or its position determination
system has to be calibrated. The determined calibration data must be
provided to a control device associated with the actuator in a suitable
manner, with a first installation or a replacement of the actuator being
possible without interfering with the control device.

[0005]It is therefore the object of the invention to provide a position
determination system of a rear axle steering actuator which can be
calibrated in a simple and reliable manner. In addition, a corresponding
method for the calibration of the position determination system should be
provided.

[0006]This object is satisfied by a position determination system or by a
method in accordance with claim 14 or claim 1 respectively.

[0007]As already initially mentioned, the method in accordance with the
invention serves for the calibration of a position determination system
of a rear axle steering actuator for a motor vehicle. The rear axle
steering actuator has an actuator element which can be driven to a
translation movement by a rotary movement of a rotor--for example the
rotor of an electric motor which is a part of the rear axle steering
actuator--and whose geometrical center position is determined by means of
a reference measurement. The position determination system includes a
linear sensor and a rotary sensor. During the calibration of the position
determination system, a piece of calibration information is generated
which includes a piece of zero point information of the linear sensor and
a piece of sector information. The measurement range of the rotary sensor
is divided into at least two sectors and the sector information
identifies that sector in which the angular position of the rotor
detected by the rotary sensor lies when the actuator element is arranged
in its geometrical center position. The generated calibration information
is stored in the linear sensor.

[0008]In other words, the position determination system to be calibrated
uses two different sensors with different measurement ranges and
measurement accuracies in order always to generate an accurate position
signal of the actuator element. The linear sensor, for example, has a
larger measurement range than the rotary sensor; however, it is less
precise with respect to its spatial resolution. A cost-effective, but
nevertheless sufficiently precise rotary sensors with respect to its
angular resolution can possibly only make a determination as to which
angular position the rotor driving the actuator element adopts. Such
rotary sensors can frequently not deliver any information with respect to
the absolute position of the actuator element. An accurate position
determination of the actuator element can be carried out by the
combination of the pieces of information of the two sensors. The
requirement for this is, however, a precise calibration of the position
determination system.

[0009]For this purpose, the geometrical center position of the actuator
element is determined by means of a reference measurement. In addition,
the calibration information already mentioned above is generated which
includes at least two pieces of partial information. The zero point
information of the linear sensor indicates which position information the
linear sensor determines when the actuator element is located in the
named geometrical center position. The sector information, in contrast,
relates to the rotary sensor whose measurement range is divided into at
least two sectors. I.e. the measurement range of the rotary sensors
corresponding to a mechanical revolution of the rotor is divided into a
plurality of sectors which cover specific angular ranges. The sectors can
generally have different sizes; however, a division into sectors of equal
size is preferred.

[0010]If the actuator element is located in the geometrical center
position, the rotary sensor delivers an angular value which can be
associated with a specific sector. The sector information thus contains
the information as to in which of the sectors the angular position of the
rotor driving the actuator element lies when the actuator element is
arranged in the geometrical center position. The advantage of the method
inter alia comprises the fact that the pieces of partial information of
the calibration information can be determined independently of one
another.

[0011]The calibration information is stored in the linear sensor. The
information applicable to the respective installed actuator is thus
always available and an updating of calibration information stored in an
external control device is not necessary.

[0012]In an exemplary installation process of the rear axle steering
actuator in a motor vehicle, the actuator element is located in the
center position, i.e. in a neutral position, which is in particular of
importance on a traveling of the vehicle straight ahead. No further
calibration activities are thereby necessary to make the system ready for
operation, such as an actuator-specific configuration of a control device
controlling the driving dynamics of the vehicle.

[0013]Advantageous embodiments of the invention are set forth in the
dependent claims, in the description and in the drawings.

[0014]In accordance with an advantageous embodiment of the method In
accordance with the invention, the respective angular range covered by
the sectors is larger than the angular resolution of the rotary sensor.
Since the number of the sensors is thus smaller than the number of the
angular positions which can be resolved per se, the sector information to
be stored is also smaller. The selection of the sector size can, for
example, be matched to the required calibration precision.

[0015]Provision can be made that the linear sensor includes a non-volatile
memory section in which at least one part of the calibration information
is stored. In other words, only the zero point information can be stored,
for example. In addition, the sector information can also be stored in
the memory section. Such a memory section is anyway provided in the
linear sensors to be considered for use so that no cost-driving separate
components are required to store the calibration information.

[0016]The sector information can also be stored in the linear sensor in
different manners. A respective preset frequency of an output signal of
the linear sensor is preferably associated with the sectors. The sector
information can be stored in the linear sensor by a selection of the
corresponding frequency of the output signal. In other words, the sector
information can be encoded in the output signal frequency by a
corresponding configuration of the linear sensor. Based on this
procedure, additional memory space for the storage of the sector
information can be dispensed with.

[0017]As already mentioned above, the geometrical center position of the
actuator element is determined by means of a reference measurement. The
reference measurement can in each case include at least one measurement
in at least two defined reference positions, with each measurement
including at least one measured value determination of at least one
sensor. A measurement can thus include a plurality of measured value
determinations by a plurality of sensors.

[0018]The reference measurement preferably includes measurements by means
of a third position sensor which is in particular based on an optical
measurement process. Such measurement processes can be carried out
particularly precisely and simply. The initially addressed problem of the
easy contamination of optical sensors is only present in the operation of
the rear axle steering actuator and not on its calibration, which in
particular takes place ex works under controlled conditions. The position
determination system is preferably calibrated by means of laser
measurements.

[0019]In accordance with a particularly simple embodiment of the
calibration process, the actuator element is moved into the geometrical
center position in its course. Subsequently, the zero point information
of the linear sensor and the sector information of the rotary sensors are
determined there.

[0020]The geometrical center position can be determined by the formation
of a geometrical average from measurements in two maximum excursions of
the actuator element. The maximum excursions of the actuator element are
in particular defined by a respective mechanical abutment. To ensure that
a maximum excursion was actually reached, the position of the actuator
elements in the respective maximum excursion can be checked by at least
one repeat position determination and/or can be determined by averaging
of at least two position determinations.

[0021]For example, the measured value determinations of the linear sensor
and/or of the rotary sensor and/or of the third position sensor are
utilized to determine the position of the actuator element in the
respective maximum excursion a plurality of times one after the other.
The position of the actuator element in the maximum excursion is in this
case a value which was confirmed by one or more repeat measurements,
i.e., when the repeat measurements coincide within the framework of a
predetermined tolerance range. Alternatively, the position of the
actuator element in the respective maximum excursion can be determined by
an averaging of the data obtained.

[0022]This "abutment search" increases the accuracy of the calibration
process and serves for the identification of stiffness situations which
possibly result in a "sticking" of the actuator before the reaching of
the actual abutment.

[0023]The reaching of the respective maximum excursion can be defined by
falling below a predetermined threshold value of the speed of the
actuator element.

[0024]In accordance with an advantageous embodiment of the method in
accordance with the invention, the sector information is determined with
reference to measured values of the rotary sensor determined in the
maximum excursions and with reference to the number of revolutions of the
rotor determined by means of the rotary sensor during the movement of the
actuator element between the maximum excursions. In other words, that
sector can be calculated in a simple manner in which the angular position
of the rotor is disposed when the actuator element is arranged in its
geometrical center position. For this purpose, the angular positions of
the rotor in the maximum excursions and the number of the revolutions of
the rotor during the movement from one maximum excursion to the other are
required. This embodiment of the method thus makes possible the
determination of the sector information without the geometrical center of
the actuator element having to be moved to.

[0025]In accordance with a further embodiment of the method, a plurality
of measurements of the linear sensor are carried out for the position
determination in different measurement positions of the actuator element.
In this respect, the precise location of the measurement position
relative to the geometrical center of the actuator element is determined
by measurements of the third position sensor. Based on the deviations of
the position determination of the linear sensor from the precisely known
measurement positions, at least one correction parameter and/or at least
one correction function is/are determined which are taken into the zero
point information of the linear sensor. The zero point information can,
for example, be interpolated or extrapolated by this sampling point
method using at least two measurements. By the comparison of the data at
the sampling points, a check can also be made whether the linear sensor
has non-linear measurement errors in the relevant measurement range.
Optionally, corresponding correction parameters/functions are determined
to compensate for these errors. They are taken into the zero point
information of the linear sensor or are themselves stored therein. This
embodiment of the method is thus a teach-in method with sampling points.
This concept is generally independent of the center position calibration,
but can be advantageously combined with it to improve the precision of
the position determination system.

[0026]As initially mentioned, the present invention also relates to a
corresponding position determination system of a rear axle steering
actuator for a motor vehicle. The rear axle steering actuator has an
actuator element which can be driven to a translation movement by a
rotary movement of a rotor. The position determination system includes a
rotary sensor and a linear sensor which are connected to the rear axle
steering actuator. The position determination system is moreover
connectable to a calibration unit for the calibration by which a
geometrical center position of the actuator element between two maximum
excursions can be determined. Furthermore, the measurement range of the
rotary sensor--which, for example, corresponds to one revolution of the
rotor--is divided into at least two sectors by the calibration unit. In
addition, a piece of calibration information can be generated by it which
includes the above-described zero point information and the sector
information. The calibration information can also be stored in the linear
sensor by the calibration unit.

[0027]In other words, the position determination system can be calibrated
by a calibration unit, for example at the manufacturer's of the rear axle
steering actuator. The calibration unit in this respect, on the one hand,
takes over the determination of the geometrical center position of the
actuator element; on the other hand, also the generation and storage of
the calibration information.

[0028]A rear axle steering actuator having the above-described position
determination system can be calibrated in a simple manner, with the
corresponding information being stored in the actuator itself, namely in
the linear sensor installed in it. Since no information has to be stored
in the rotary sensor, additional memory means and their connection to
further control devices are omitted there.

[0029]The respective angular range covered by the sectors can preferably
be divided such that it is larger than the angular resolution of the
rotary sensor.

[0030]Provision can further be made that the linear sensor includes a
non-volatile memory section in which at least one part of the calibration
information can be stored.

[0031]In accordance with a further embodiment of the position
determination system, the linear sensor can be configured such that the
sector information is encoded by a frequency of an output signal of the
linear sensor associated with the identified sector. A part of the
calibration information--the sector information--is thereby provided by a
characteristic parameter of the output signal--its frequency. For
example, the device controlling the rear axle steering actuator obtains
the zero point information of the linear sensor in a conventional manner,
whereas the sector information of the rotary sensor can be taken from the
frequency of the signal. Two different pieces of information thus do not
have to be read out of and transmitted from one or more memory sections,
which considerably simplifies the information transmission.

[0032]Provision can be made that the calibration unit includes a position
sensor which is in particular based on an optical measurement process,
with the position of the actuator element relative to a reference point
being able to be determined by means of the position sensor.

[0033]The calibration unit can additionally be made such that the
above-described correction parameters/functions can be determined by
means of sampling points.

[0034]The present invention further relates to a method for the operation
of a rear axle steering actuator for a motor vehicle in accordance with
at least one of the above embodiments, with the actuator element being
moved in a first step into a position defined by the zero point
information of the linear sensor for the positioning in a neutral
position--for example, in a position for a traveling of the vehicle
straight ahead. Subsequently, the actuator element is moved in a second
step until the angular position of the rotor detected by the rotary
sensor lies in the sector identified by the sector information. This
step-wise procedure is of advantage in particular when the linear sensor
has a poorer position resolution capability than the rotary sensor--which
results in the latter from its angular resolution capability and from the
amount of the translation movement of the actuator element which is
generated by a revolution of the rotor--. The linear sensor indicates the
absolute position of the actuator element which cannot be determined by a
simple rotary sensor since it can only indicate the angular position of
the rotor. In the second step, the rotor is turned so far, starting from
the position reached in the first step, until its angular position comes
to lie in the sector identified by the sector information. The extent to
which the neutral position reachable by this process deviates from the
exactly determined geometrical center position thus depends inter alia on
the number of sectors. With a suitable coordination of the tolerances of
the linear sensor, of the translation amount of the actuator element per
revolution of the rotor and of the resolution capability of the rotary
sensor, it is accordingly possible to travel to a neutral position which
deviates from the geometrical center position as a maximum by the angular
range of a sector or by the movement of the actuator element generated on
a corresponding rotation of the rotor.

[0035]A preferred embodiment of the method for the operation of a rear
axle steering actuator provides that the actuator element is moved in the
second step until the bisectrix of the identified sector is reached,
whereby the maximum deviation of the neutral position from the
geometrical center position amounts to a maximum of half the angular
range covered by the sector.

[0036]The invention will be described in the following purely by way of
example with reference to advantageous embodiments and to the drawings.
There are shown:

[0038]FIG. 2 a diagram for the explanation of the division of the
measurement range of the rotary sensor into sectors;

[0039]FIG. 3 a diagram for the illustration of the calibration process.

[0040]FIG. 1 shows a rear axle steering actuator 10 which has an electric
motor 12. A stator 14 of the electric motor 12 is fastened to the inner
side of a housing 13 of the rear axle steering actuator 10--in the
following also briefly called the actuator 10--composed of a plurality of
individual parts. A rotor 16 of the electric motor 12 associated with the
stator 14 is seated on a tubular shaft 18. It merges directly at one side
into a spindle nut 20 which is firmly connected to it. Alternatively, the
shafts 18 and the spindle nut 20 can be made in one piece.

[0041]The tubular shaft 18 with the spindle nut 20 is journaled on the
side of the spindle nut 20 in a first bearing 22 and on the side remote
from the spindle nut 20 in a second bearing 22' in the housing 13 of the
actuator 10. The bearing 22 has particularly thick dimensions, is here
made as a double ball bearing, for the taking up of substantial
journaling forces. The spindle nut 20 has a thread 24 of small diameter
and small pitch over a part of its axial extent. Adjoining the thread 24,
the spindle 20 has collars 26, 26' of larger diameter than the diameter
of the thread 24 in both axial directions. The spindle, which is marked
overall by the reference numeral 28, comprises a threaded spindle 30 of
smaller diameter, whose thread cooperates with the thread 24 of the
spindle nut 20, and a spindle part 32 of larger diameter adjoining it
directly at the right in FIG. 1. In other words, the section of the
spindle 28 having the thread has a smaller diameter--in particular a
smaller outer diameter--than the sections of the spindle 28 adjoining the
threaded section. The spindle part 32 has a center longitudinal bore 34
at the side facing the threaded spindle 30 into which the threaded
spindle 30 is inserted and is firmly screwed. The spindle part 32 is
supported at its right hand part in FIG. 1 in a sliding bearing 36 with a
security against rotation and ends in a connector piece 38 which is
firmly connected to it by means of a bolt 40. A steering link 44 leading
to a wheel and connected by a pivot axle 42 engages at the connector
piece 38.

[0042]A spindle part 32' of larger diameter likewise directly adjoins the
side of the threaded spindle 30 at the left in FIG. 1. The spindle part
32' is made in one piece with the threaded spindle 30 in the embodiment
shown. A shoulder 46 is arranged at the transition between the spindle
part of larger diameter 32' and the threaded spindle 30. The end of the
spindle part 32 remote from the spindle nut 20 is--analog to the
embodiment of the right hand section of FIG. 1--supported in a sliding
bearing 36' with a security against rotation and has a connector piece
38' for the connection of a left hand steering link 44'.

[0043]A respective bellows 48, 48' is provided between the housing 13 and
the connector pieces 44, 44' to keep dirt away from the sliding bearings
36, 36'. A linear path sensor 50 fastened to the housing 13 is provided
at the left hand spindle part of larger diameter 32'; a rotary sensor 52
is provided at the side of the tubular shaft 18 remote from the spindle
nut 20. The two sensors 50, 52 form signals for the control, not shown,
of the actuator 10. A current supply 54 for the operation of the actuator
10 is only shown by indication.

[0044]To enable an actuating position of the rear axle steering actuator
10, the rotor 16 of the electric motor 12 is set into rotation. The rotor
16 drives the spindle nut 20 via the tubular shaft 18. This rotary
movement is converted into a translation of the spindle 28 via the thread
24 and the threaded section of the spindle 28, said translation resulting
in a corresponding excursion of the steering links 44, 44'.

[0045]So that the vehicle dynamics can be advantageously influenced by the
rear axle steering, it is of essential importance that the position of
the spindle 28 relative to the housing 13 of the actuator 10 fixedly
installed in the vehicle is always accurately known. This requires a
calibration of the sensors 50, 52 which, on the one hand, themselves have
tolerances; but which, on the other hand, cannot be completely perfectly
positioned on their installation in the actuator 10 so that their
internal zero point does not necessarily have to coincide with the
geometrical center position of the spindle 28 relative to the housing 13.
In addition to the calibration information of the path sensor 50, a piece
of calibration information relating to the rotary sensor 52 is also
detected and stored since the zero point information of the path sensor
50 does not necessarily have to correspond to the absolute zero position
of the rotary sensor 52. The calibration information of the total system
is composed of the two named pieces of information.

[0046]A further important aspect relates to the requirement of the
replaceability of the actuator 10. The demand is made on car makers that
any desired control unit has to be able to move to a defined position of
the actuator 10 after the activation of the rear axle steering actuator
10. I.e. the calibration values of the installed actuator 10 should not
be stored in an external driving dynamics control device since this would
require a further memory section in the control device and additional
worksteps in the installation or on a replacement of the actuator 10.

[0047]A general solution of this problem comprises the fact that the path
sensor 50 is calibrated to a zero point position which corresponds to the
geometrical center position of the spindle 28 between two abutments which
bound the maximum excursion of the spindle 28 symmetrically in both
directions, for example so that the wheels of the vehicle do not knock in
the wheel housing. A corresponding piece of zero point information is
stored in the path sensor 50. For this purpose, corresponding memory
space is available as standard to the path sensor.

[0048]However, there is a further problem in that the described zero point
determination is not sufficiently precise since the resolution of the
path sensor 50 is not sufficiently high. Generally, the accuracy can be
improved in that the angular position of the rotor 16 which is detected
by the rotary sensor 52 is taken into account in the calibration. This is
possible when the rotary sensor 52 can detect a difference of angular
positions of the rotor 16 which corresponds to a translation movement of
the spindle 28 which is smaller than the resolution capability of the
path sensor 50. The measured value of the rotary sensor 52 is registered
for the calibration when the spindle 28 is located in the geometrical
center position which was determined, for example, by an external
positioning system. The registered measured value is taken into account
on the generation of the calibration information of the actuator 10. In
particular optical measurement processes by means of lasers are suitable
for the determination of the geometrical center position, which can, for
example, take place ex works at the manufacturer's of the actuator.

[0049]The information of the rotary sensor 52 required for the generation
of the calibration information is, however, not stored in the rotary
sensor 52, but rather in the linear path sensor 50. A memory section for
the storing of the named information in the rotary sensor 52 can thereby
be omitted. The common storage of the calibration information of the path
sensor 50 and of the rotary sensor 52 moreover simplifies the
communication of the actuator 10 with an external control device.

[0050]To keep the memory requirements of the information of the rotary
sensor 52 obtained by the calibration low, the measurement range of the
rotary sensor is divided into a plurality of sectors, as is shown in FIG.
2.

[0051]FIG. 2 shows a diagram of the sector division of a rotary sensor 52
which is based on the principle of a sine-cosine sensor. With such a
sensor, a mechanical revolution of 360° of the component to be
observed--here the rotor 16--corresponds to two sensor periods of a total
of 720°, i.e. 1° of mechanical rotation corresponds to
2° of the electric signal. A suitable rotary sensor 52, for
example, has a resolution of ±3.6° mechanically
(±7.2° electrically), which corresponds to a resolution of
±20 μm for a thread stroke of the thread 24 of approximately 2 mm.
This resolution is much higher than the resolution of a conventional
linear path sensor 50 which usually amounts to ±175 μm in the
region around the sensor center.

[0052]Since the exploitation of the generally possible resolution
capability of the rotary sensor 52 is not necessary for a sufficiently
good calibration and since the memory depth required for the storage of
the calibration value should be minimized, the measurement range of the
rotary sensor 52 is divided into a plurality of sectors which are
associated with specific angular ranges of a mechanical revolution of the
rotor 16. Expediently, these sectors are of equal size. This division of
the measurement range substantially represents a reduction of the
resolution capability of the rotary sensor 52 which accompanies a
reduction of the memory requirement for the storage of the calibration
value. The optimum between the required calibration precision and the
available memory depth can be achieved by a suitable choice of the
angular ranges or of the number of sectors.

[0053]FIG. 2 shows such a division of the measurement range of the rotary
sensor 52. As already described above, a mechanical revolution of the
rotor 16 corresponds to two sensor periods A, B. FIG. 2 shows a division
of a mechanical revolution, whereas the angular indications of the
sectors in the table shown above the diagram relate to two sensor periods
with a total of 720°. A decision can be made on the basis of the
sufficiently high resolution capability of the path sensor 50 in which of
the sensor periods A, B the angular position of the rotor 16 lies. A
mechanical revolution is therefore effectively divided into four sectors
which are designated by Sec 0 to Sec 3.

[0054]For the calibration of the position determination system of the
actuator 10 which includes the linear path sensor 50 and the rotary
sensor 52, it is thus not the value Mi of the exact angular position
of the rotor 16 which is stored when the spindle 28 is located in the
previously determined geometrical center position. It is rather the case
that only a piece of information is stored with respect to that sector in
which the angular position Mi of the rotor 16 corresponding to the exact
center position comes to lie. This is the sector 3 in the case shown.

[0055]Since a mechanical revolution of the rotor 16 was divided into four
sectors in the example shown, the sector information can be encoded by
two bits so that the information of the rotary sensor 52 to be stored has
only a low memory requirement. The present embodiment has as a special
feature that the sector information is, however, not stored in a specific
area of the memory section of the path sensor 50, but is rather encoded
by a selection of the output signal frequency of the path sensor 50. The
table already mentioned above shows the association of the individual
sectors to specific frequencies of the output signal. In the case shown,
the rotor 16 lies in the geometrical center position of the spindle 28 in
the angular position Mi which falls in the sector Sec 3. The path
sensor 50 is therefore configured within the course of the calibration so
that its output signals are transmitted at 2000 Hz. This information can
be detected and utilized simply by an external control device.

[0056]In the following, the course of the calibration method will be
explained in more detail with reference to FIG. 3.

[0057]The control of the calibration process is carried out by an external
calibration unit. This includes, for example, a laser sensor which is
used for the determination of reference positions. The procedure during
the calibration process is shown in FIG. 3.

[0058]FIG. 3 shows the position of the spindle 28 as is represented in the
data space of a laser sensor for the absolute determination of the
spindle position--designated by L--and of an SC rotary sensor
(sine-cosine rotary sensor--designated by SC. Starting from a start point
S, which can correspond to any desired position of the spindle 28, the
spindle 28 is moved by rotation of the rotor 16 up to the reaching of a
first abutment position P1. Subsequently, the direction of rotation of
the rotor 16 is reversed, with initially a translation movement of the
spindle 28 not yet being recorded. Only from point P2 does the spindle 28
start to move in the other direction. The path from the abutment P1 to
the point P2 only detected by the rotary sensor 52 is designated as a
hysteresis which is shown clearly overdrawn in FIG. 1.

[0059]The spindle 28 is moved so long until a second abutment position P3
has been reached. At the points P1 and P3, the measured values L1 and L3
respectively of the laser sensor and the angle data of the rotary
sensor--SC1 and SC3--are detected.

[0060]The geometrical center position of the spindle 28 can be precisely
determined from the measured values L1 and L3. The sector can already be
determined from the values for the angular position of the rotor 16 in
the abutments P1, P3 (SC1 and SC3 respectively) in which the angular
position Mi of the rotor 16 corresponding to the geometrical center
position lies if, in addition, it is known how many revolutions the rotor
16 has made during the movement of the spindle 28 from the first abutment
position P1 to the second abutment position P3.

[0061]It is naturally also possible to determine the sector in which the
angular position Mi lies in that the spindle 28 is moved into the
geometrical center position determined by the laser sensor and the value
of the rotary sensor 52 is read there as can also be practiced in an
analog manner for the calibration of the linear path sensor 50.

[0062]During the operation of a vehicle, it is frequently necessary that
the rear axle steering actuator 10 adopts a neutral position so that, for
example, an exact driving straight ahead is possible. To move to the
neutral position, the spindle 28 is moved so long until the position
determined by the path sensor 50 coincides with the stored zero point
information of the linear path sensor 50. The spindle 28 is thus already
in the correct half-turn of the rotary sensor 52 so that the ambiguity
(see FIG. 2) existing due to the presence of the two sensor periods A, B
can be resolved. The spindle 28 is now moved further for so long until
the rotary sensor 52 outputs a measured value which lies in the sector
defined by the output signal frequency of the path sensor 50. It can be
seen directly from FIG. 2 that the maximum angle deviation between the
angular position Mi of the rotor 16 corresponding to the geometrical
center position and the angular position of the rotor 16 amounts to
45° in the neutral position moved to.

[0063]To reduce this maximum deviation, on a movement to the neutral
position of the spindle 28, a stop is not made on the reaching of the
correct sector, but rather the rotor 16 driving the spindle 28 is rotated
so far until the rotary sensor 52 outputs a value which corresponds to a
bisectrix M of the corresponding sector. The maximum angular position
deviation of the rotor 16 from the angular position Mi corresponding
to the geometrical center position of the spindle 28 thus halves and now
amounts to a maximum of 22.5°. The bisectrix M associated with the
individual sectors Sec 0 to Sec 3 can be seen from the last column of the
table in FIG. 2.

[0064]The neutral position of the spindle 28 can already be moved to more
precisely by a division of a revolution of the rotor 16 into four
measurement range sectors of the rotary sensor 52 than while using only a
linear path sensor 50 of the kind described above. The information
required for this purpose does not require any further memory space. An
increase in the number of sectors leads to a further precision of the
described procedure.

[0065]It was described above with reference to FIG. 3 that the abutment
positions P1, P3 are of fundamental importance for the carrying out of
the reference measurement. However, stiffness situations frequently occur
which can simulate the achieving of the abutment. To prevent artifacts
thereby arising, the spindle 28 is moved so long in a direction until its
measured speed falls below a threshold value. After falling below this
minimal value, it is assumed that the abutment has actually been reached.
The spindle 28 is then moved in the opposite direction by a specific
amount and the abutment is "searched" again. This procedure is repeated
for so long until the respectively determined abutment positions of three
passes--within a predetermined tolerance range--deliver coinciding
results. Alternatively or additionally, an averaging of the results can
be carried out and/or two or more than three passes to find the abutment
can be carried out.

[0066]To improve the position determination, a reference curve
characterizing the path sensor 50 can be prepared by means of sampling
points whose position is precisely known by the laser measurements
described above. For this purpose, the position data determined at the
sampling points by the linear path sensor 50 are compared with the known
position data. Correction parameters can be determined by the comparison
which can be used for the calibration of the path sensor 50. The sampling
points can in particular be spaced closer to one another in the region
around the geometrical center position than, for example, in the
proximity of the abutment position P1, P3 to make the correction
particularly precise here.

[0067]The described concepts are generally not restricted to the use with
respect to rear axle steering systems. Their use can also be considered
in connection with other actuators and systems--in particular in the area
of a driving dynamics regulation of motor vehicles--in which the
addressed problem is relevant, such as wobble stabilizers or systems to
influence the wheel suspension geometry.